High voltage transients such as those caused by electrostatic discharge can have adverse effects on electrical circuitry. For example, the high voltages may break down dielectrics causing arcing between various portions of circuits. Passive components can fail in a variety of ways. The high voltage arcing may locally destroy dielectrics within capacitors causing them to develop low series resistance or they may vaporize or fuse materials used to fabricate resistors. The energies, currents and voltages associated with electrostatic discharge events may cause semiconductors to fail. Sufficient energies are present to melt bonding wires and local areas of thin semiconductor material. These and other events cause semiconductors to fail or to behave abnormally.
The detrimental effects of electrostatic discharge events are accentuated as circuit densities are increased and as parts are miniaturized. Then distances between the conductors decrease so that as electrical field intensity increases the ability of materials to behave as insulators is reduced. Moreover the energy dissipated by a discharge is absorbed by a smaller device thereby creating a greater temperature rise than in the case of a large device. While earlier concerns were to limit transients to several thousand volts it is now important to limit transients to relatively low values, say, a few hundred volts.
To overcome these difficu1ties it is known to protect semiconductors and other circuits by limiting currents with resistors while providing alternative discharge mechanisms and paths. Specifically, it has been proposed to incorporate a spark gap between adjacent conductive paths on a printed circuit board to discharge high voltages at the input terminals of circuit devices to avoid the destructive dissipation of energy at sensitive portions of the circuit. In such devices, however, it is desired that the transient voltages be controlled to a low level and that the discharge voltage be consistent over a period of time and stable in adverse operating environments. It is also desired that any current limiting resistor be protected so that it does not become bypassed or damaged. It is a further requirement that the protective device not be destroyed or debilitated by a transient high voltage but rather it should remain operative to maintain circuit protection even after repeated applications of high voltage transients. It is known to cover spark gaps with a dielectric coating, however, this presents two problems; first the breakdown potential of the spark gap is greatly increased as compared to the breakdown potential in air and moreover it can be expected that the dielectric would be destroyed at least in part by a single discharge so that its effectiveness would be reduced or lost.